Typical television broadcasts do not allow personalization of television content to a viewer's profile. The standard television broadcast provides only one variant of every channel. The channel is selected by the viewer and the reception equipment (whether a television, a set-top-box, or any means of reception) selects the video and audio for that channel from the broadcast material. While this system allows the viewer to select their favorite channel or show from the available set, the individual viewer will be watching exactly the same content as everyone else that selects that channel. Due to the fact that channels are created to attract a wide range of viewers, viewers typically have different preferred channels. Disadvantageously, this is particularly evident when a program being broadcast on a channel is interrupted by a commercial advertisement that does not appeal to the interests of the viewer. The inevitable result is that the viewer will switch to another channel to avoid watching that particular commercial advertisement. It would be advantageous to personalize channels to be viewed by a viewer tailored to their particular interests and personal situations. For example, inclusion of personalized commercial advertisements will make such messages more relevant to the viewer, reducing the desire to change the channel being viewed.
A method of creating personalized messages is disclosed in co-pending U.S. patent application Ser. No. 09/841,465 filed on Apr. 24, 2001, which is incorporated herein by reference. One technique which can assist in the process of assembling personalized messages is the ability to switch rapidly between multiple data streams (such as audio and/or video feeds) that are received simultaneously, in order to assemble the message in real time, possibly as the message is being viewed by the end user.
However, switching between multiple data streams is problematic. One problem is that switching typically is not instantaneous. With present technology, it is difficult to switch from one high-bandwidth digital data stream to another without missing some data in at least one of the streams. This is true no matter what the type of data in the stream is, including audio, video, graphics, etc., or the type of switch, whether hardware (such as an integrated circuit), software or a combination of the two. Also, timing the switch point to minimize data interruption is very difficult. Switching between two streams typically results in artifacts due to loss of data or sometimes even introduction of erroneous data,. For multimedia (such as television) signals, switching introduces very audible and visible artifacts in the sound and picture.
An example of this switching problem appears in MPEG based digital television. MPEG defines standards for digital television signals. The MPEG standards include the capability for compressing, coding and transmitting high-quality multi-channel, multimedia signals over a variety of broadband networks. MPEG encodes media signals as sequences of frames, and switching between separate sequences of frames multiplexed in, e.g., MPEG-2 transport streams takes a non-zero amount of time, and is usually partly executed in hardware and partly in software. In addition, only at certain moments in time a switch is actually allowed due to dependencies between groups of data in MPEG (frame accurate switching is required). To illustrate this further, Fig. 1 shows switching and decoding components of an example digital television receiver 20. The transport stream 48 carrying multiple encoded data streams enters the demux (demultiplexer) 32. This demux 32 serves as the switch, by selecting which video and which audio data stream in the multiplexed transport stream 48 to pass on. These streams are then decoded respectively by a video decoder 42 and an audio decoder 44 (with buffers 38 and 37 for the encoded data between the demux 32 and the decoders 42 and 44). The results of the decoding are a stream of video frames 40 and audio samples 38, which can then be sent to display and audio equipment. The decoder is controlled by a receiver controller 46, which typically uses a microprocessor and software.
Normally, when switching between different video and audio streams within the Transport Stream 48, the receiver controller 46 first mutes/blanks the affected decoder, (as shown by arrows 43 and 45, then switches the Demux 32 settings and then unmutes/unblanks the decoder(s). This will present a moment of silence/black to the viewer, it will never be a seamless switch for the viewer.
In an attempt to get a seamless switch, the mute/unmute sequence may be skipped. Now, however, the results depend on the exact moment of the switch with respect to the incoming data from the transport stream. Digital compression and transmission creates interdependencies between groups of video frames because of encoding and packaging and groups of audio samples because of packaging. Only at certain points within each data stream within the transport stream is it possible to switch out of the current stream without having visible and/or audible artifacts (safe exit point). Similarly only at certain points within each data stream within the transport stream, it is possible to switch into that stream without having visible and/or audible artifacts (safe entry point). The requirement of exactly hitting a combination of safe exit and safe entry point make the seamless switch very difficult. In addition to this, the decoders 42, 44 are typically the only devices in the receiver 20 that can detect the right switching moments, while the demux 32 is the device that must be switched. Because of extensive data buffering between the demux and the decoders, detection by the decoder is of no use to determine the right moment to switch the demux.
One solution would be to build new receivers with specialized hardware and software (possibly including additional buffering at several locations in the receiver) to support seamless switching. However, this solution increases the cost and complexity of receivers, and can not take advantage of the existing receivers on the market.